The efferent innervation of the avian cochlea.

The efferent auditory innervation of the budgerigar (Melopsittacus undulatus) was investigated by means of acetylcholinesterase staining. The course of the efferent fibres is described: the single terminal fibre regularly branches out to innervate several hair cells. Three types of efferent, axosomatic synapses could be distinguished: small outer hair cells bear large cup-like efferent synapses, intermediate hair cells with cone-shaped synapses, and tall inner hair cells with small, knob-like synapses. In the basal part of the cochlea the small hair cells with heavy efferent endings predominate, while in the apex only tall inner hair cells with small efferent terminals exist. There are more intermediate hair cells in the apical area than in the basal area.